MAME stands for Multiple Arcade Machine Emulator. When used in conjunction with an arcade game's data files (ROMs), MAME will more or less faithfully reproduce that game on a PC. MAME can currently emulate over 3000 unique (and over 5400 in total) classic arcade video games from the three decades of video games - '70s, '80s and '90s, and even some from the current millennium.

A emulator made to recreate the experience of arcade games in software,with the intent to preserving old games from being forgotten.The name stands for: Multiple Arcade Machine Emulator.MAME was created by Nicola Salmoria on February 5,1997.To play the games on MAME on a PC or Mac you have to get a set of files called ROM files.Most people hide the Windows operating system when its in a arcade cabinet.People either make their own cabinet or buy a second hand one to run MAME on.Owning MAME is legal in most countries,but not to be sold or distributed.

Software that emulates (mimics the behavior of) many arcade systems, though with a massive overhead.

In plain English, if you have a copy of an arcade game, you can (theoretically) play it in your PC using MAME. Arcade games are usually stored in ROM chips, so such a copy is called a ROM image or simply "rom". New arcade systems use harddisks, so in that case you have a copy of the harddisk, usually called a "chd" file. Mimicing the behavior of an arcade system is done by translating the instructions from the "machine language" of the arcade system to the "machine language" your PC uses.

The only catch is that, as mentioned, MAME has huge overhead and thus is slow. Even if the original arcade system run at a measly 30Mhz, your 2-3GHz CPU may struggle to cope.

MAME developers claim that such overhead is neccessary to make the result as faithfull to the original as possible, but truth is, MAME could be faster without losing any accuracy, for example skipping step-by-step emulation when not neccessary.
But since the stated goal of the MAME project is to preserve the arcade legacy for future generations and not make arcade games playable right now, theoretically the project has achieved it's goal.

On a practical level, 2D games will run fine, but for 3D games (even early ones), you 'd better look elsewhere, such as Nebula, Kawaks, Calice, Zinc, FinalBurn Alpha, VivaNonno, Raine and other fast emulators.

One more detail: Use MAMEUI for Windows, and SDLMAME for OS X.

I just played the arcade version of Pacman in my PC using MAME

I can't believe MAME needs a 3GHz CPU to run Ridge Racer 1 in my PC. The original arcade system had a 24.5Mhz CPU!